1. Field of the Invention
The present invention relates to single crystal structures and, more particularly, is directed towards a method for forming single crystal structures which are replicas of a starting single crystal of specified composition. The resultant replica single crystal structures, for example, are designed for fabricating solar cells or infrared detectors therefrom after further processing.
2. The Prior Art
Single crystal wafers formed from silicon, germanium or gallium arsenide serve as suitable materials for fabricating energy-efficient solar cells. Texture-etching of such wafers results in improved efficiency; the reason for such improvement is that the many facets and the differing angles and spacings among the facets of a texture-etched surface significantly increase the amount of solar energy absorption in a solar cell so formed. Presently, such single crystal wafers are grown in furnaces and the wafers then are texture-etched. This furnace-grown procedure is both inefficient and wasteful, hence expensive. The procedure is inefficient for it requires a great deal of energy to grow, in a furnace, large-grain single crystal wafers needed for making solar cells. The procedure is wasteful since about less than half of the wafers so grown pass the rigid requirements for service as base materials being fabricated into solar cells.
Recently, attempts have been made to grow large-grain crystal sheets from amorphous silicon films deposited on substrates formed with a one dimensional artificial surface-relief grating. Such a one dimensional surface-relief grating includes square-wave and sawtooth channels formed in the surface of the substrate, which channels comprise periodic arrays of facets that meet at 90.degree. angles to each other. See Technical Report 533 of M.I.T. Lincoln Laboratory Ltd. Dec. 5, 1978, D. C. Flanders, "Orientation of Crystalline Overlayers on Amorphous Substrates by Artificially Produced Surface Relief Structures." After deposition, the films are scanned with a laser beam to effect crystallization. The scanned films exhibit a one dimensional crystalline orientation as determined by the surface-relief grating. The resultant sheets are but small-grain polycrystal silicon sheets, unusable for fabricating solar cells therefrom. These attempts thus have failed to provide an acceptable fabrication technique for growing large-grain single crystal sheets of the type required to make solar cells therefrom.